The statistical analysis was performed using JASP open-source software (version 0.13.1, Amsterdam, Netherlands). Before comparing each dependent variable, the normality of the data distribution was confirmed with the Shapiro-Wilk test. The effect of age (young vs old) and pulse duration (RC_0.05, RC_0.2 and RC₁) on the H-reflex and M-wave variables were investigated using two-way ANOVAs with repeated measures for pulse duration (age × pulse duration). The effect of age and pathway (H reflex vs M wave) was assessed on the SDTC and the response threshold using two-way ANOVAs with repeated measures for the pathway (age × pathway). When sphericity assumptions were violated (Mauchly’s test of sphericity), the Greenhouse-Geisser correction was applied. When appropriate, post-hoc tests with the Bonferroni correction were used to identify differences between means. The effect of age on the H-reflex and M-wave latency was investigated using Student t-tests. The level of statistical significance was set at p ≤ 0.05 for all comparisons. Values are expressed as mean (SD) in the text and tables and mean (SEM) in the figures.

M_max amplitude was lesser in old than in young adults (age effect; p = 0.002; Figure 3A) for all pulse durations (pooled data; young: 5.0 mV; old; 2.8 mV) and did not change with pulse duration (p = 0.23; Table 1. A significant age × pulse duration interaction was observed for I_M50 (p = 0.027). The associated post-hoc analysis indicated a greater I_M50 for RC_0.05compared with RC_0.2 and RC₁ in both age groups (p<0.001), and a greater I_M50 in old compared with young adults for RC_0.05(p<0.001; Figure 3C). M_max latency was similar in young and old adults [young: 0.08 (0.02) ms.cm-1; old: 0.09 (0.02) ms.cm-1; p = 0.55].